This invention relates to galvanic cells in which the electrochemical reactants are an alkali-metal e.g. sodium, as the anode and sulphur as the cathode, anode and cathode compartments being separated by a solid electrolyte which is an alkali-metal ion conductor (in the case of sodium-sulphur cells this may be a poly-crystalline ceramic conventionally known as Beta-alumina) and in which respective current collecting poles are associated with the anode and cathode compartments. The cell is operated at a temperature of about 350.degree. C so that the electrochemical reactants are in the molten state. During discharge of the cell polysulphides of the alkali metal are formed in the cathode compartment.
The physical shape of the cell may be of tubular of flat-plate form. When of tubular form, the cell comprises outer and inner tubes spaced from each other. The outer tube may be of electronically conducting material so that it constitutes one current collecting pole of the cell, the inner tube comprising the solid electrolyte. Thus the space between the two tubes defines either the anode or cathode compartment as desired and the space within the electrolyte tube then constitutes the cathode or anode compartment. A current collecting pole extends into the electrolyte tube. When the cell is of flat-plate form, electronically conducting plates close the ends of a shallow cell housing and constitute the current collecting poles and an electrolyte plate extends across the housing to divide it into anode and cathode compartments. A series of flat-plate cells can be stacked end to end to form a layer-type battery and in this case a single electronically conducting plate can be used between each pair of adjacent cells to act as a bi-pole; such an arrangement is described in British Pat. No 1,344,069.
For efficient cell operation the current collecting poles must have a low electrical resistance. However the severe corrosion conditions associated with the sulphur and polysulphides in the cathode compartment at the cell operating temperature preclude the use of conventional low resistance metals for the current collecting pole in contact with these materials. Carbon or graphite offers itself as a material for use as a possible current collecting pole for this corrosive environment but suffers from the disadvantage that it has a relatively high resistance which would result in ohmic losses in the pole causing lowering of the efficiency of the cell and a reduction of the available power. It would also result in a distribution of potential along the pole and hence a heterogeneous distribution of reaction rate throughout the cathode compartment during both the charge and discharge modes and this could have a serious adverse effect on the cyclic behaviour of the cell. Another proposal is to use an electronically conducting ceramic but this suffers from the same disadvantages as carbon and graphite.